Circuit for protecting a current limiting resistive device from excessive current



Aug. 5, 1969 J. F. SPOFFORD ET 3.460.126 CIRCUIT FOR PROTECTING A CURRENT LIMITING RESISTIVE DEVICE FROM EXCESSTVE CURRENT Filed May 6, 1966 i A? a jgvzce 47 f3 fl/ar/w United States Patent US. Cl. 340253 5 Claims ABSTRACT OF THE DISCLOSURE A bypass circuit protects a current limiting resistance lamp from damage due to excessive current from a current source connected through the lamp to a device under test when the device is short-circuited. The bypass circuit includes a thermistor and the coil of a relay both connected in series across the lamp. A normally open relay contact pair is connected across the thermistor and an alarm is connected across the coil and a normally closed relay contact pair is connected in series with the lamp. A predetermined increase in current between the source and the device actuates the alarm and, after a predetermined time, energizes the relay through the thermistor operating the contacts to shunt the thermistor, and cut off the lamp to bypass the lamp and protect it from damage.

This invention relates to a protector circuit and more particularly to a protector circuit having a bypass circuit responsive (1) to a predetermined increase in impedance of a resistive device and (2) to a predetermined decrease in impedance of a thermistor connected in the bypass circuit.

There are many industrial applications where it is necessary to limit the magnitude of test current applied a circuit module under test. One way of limiting the magnitude of the test current is to connect the circuit module to a resistive device, such as a resistive lamp, which has an impedance which increases with the amount of current flowing through it. However, many times such a current limiting resistive device is expensive and can be damaged by excessive current flow over a prolonged time duration.

It is therefore an object of this invention to provide a new and improved protector circuit.

Another object of this invention is to provide a current responsive protector circuit for bypassing the resistive de-' vice when a predetermined magnitude of current flows through the device for a predetermined period of time.

Still another object of this invention is to provide a protector circuit responsive to a predetermined increase in impedance of a resistive device being protected and a corresponding predetermined decrease in impedance of a variable resistive element connected in parallel with the device being protected.

With these and other objects in view, the present invention contemplates a bypass circuit for protecting a current limiting resistive device having an impedance which increases with the amount of current flowing through it. This current limiting resistive device is connected in series with a source of test current to limit the magnitude of the test current applied to the circuit module being tested. When a short circuit exists in the circuit module, a large current flows from the source of test current through the current limiting resistive device to greatly increase the impedance of the current limiting resistive device. This increases the magnitude of a portion of the test current flowing from the current source through the bypass circuit which includes a relay and a second resistive element hav- 3,460,126 Patented Aug. 5, 1969 ing an impedance which decreases with the amount of current flowing through it. When the impedance of the current limiting resistive device increases to a predetermined magnitude and the impedance of the second resistive element decreases to a predetermined magnitude after a predetermined time delay, the relay operates to disconnect the current limiting resistive device and to complete another bypass circuit connected around the second resistive element to greatly increase the test current in the bypass circuit, and terminates that portion of the test current flowing through the resistive device thus protecting the resistive device from excessive current.

These and other objects, aspects and advantages of this invention may be appreciated from the following detailed description and from the accompanying drawing which is a schematic diagram of a test circuit having a bypass circuit for protecting a current limiting resistive device in accordance with the principles of the present invention.

Referring generally to the figure, there is shown a circuit for protecting a resistive device or resistive lamp 11 from excessive current. The resistive device 11 is of commercial manufacture and is characterized in having an impedance which increases with the magnitude of the current flowing through it. This resistive device 11 interconnects a source of negative potential 12 through a normally closed circuit breaker 13, a junction 14, and a junction 15 to a terminal 16. A movable contact 17 connects the terminal 16 to one of a series of input terminals 18, 19, 21, 22 and 23 of a circuit module 24 under test for short circuits.

Each of these input terminals 18, 19, 21, 22 and 23 of the circuit module 24 is connected through a circuit path schematically represented as a resistance 26, 27, 28, 29 or 31 to an output terminal 32, 33, 34, 36 or 37. Each of these circuit paths may include various combinations of electrical components such as transistors, inductors and capacitors. A second movable contact 38 selectively connects a source of ground potential 39 to one of the output terminals 32, 33, 34, 36 or 37. The circuit module 24 may be tested for short circuits between the circuit paths connecting each terminal 18, 19, 21, 22 or 23 to its output terminals 32, 33, 34, 36 or 37. Thus, the movable contact 17 may connect the terminal 16 to the input terminal 22, while the movable contact 38 may connect the source of ground potential 39 to the output terminal 33 to test for a short circuit between the circuit path between input terminal 22 and output terminal 36 and the circuit path between output terminal 33 and input terminal 19. A flow of current between the input terminal 22 and the output terminal 33 indicates that such a short circuit exists in the circuit module 24.

To protect the resistive device 11 from excessive current, a bypass circuit is connected between the junctions 14 and 15. A relay coil 41, responsive to a predetermined magnitude of current flowing through it, and an alarm 42 are connected in parallel between the junction 14 and a junction 43. A second resistive element 44, such as a thermistor, having an impedance which decreases with the magnitude of the current flowing through it is connected between the junction 43 and the junction 15. A pair of normally open contacts 46 of the relay coil 41 is connected in parallel with the resistive element 44 between the junction 43 and the junction 15. A pair of normally closed contacts 47 of the relay coil 41 are connected in series with the current limiting resistive device 11 between the junction 14 and the junction 15. Contacts 47 are not necessary when the impedance of the resistive device 11 is much greater than the impedance of the parallel comb ation of the relay coil 41 and the alarm 42.

Considering now the operation of the protector circuit, when a short circuit exists in the current module 24, a large current flows between the output terminal 33 and the current flows from the ground 39, throughthe'mova'ble contact 38, the output terminal 33 of the circuit module- 24, the short circuited circuit module 24, the input terminal 22, the movable contact 17, the terminal 16, the junction 15, the pair of normally closed contacts 47, the resistive device 11, the junction 14, and the normally closed input terminal 22 of the circuit module 24. Thus, a large circuit breaker 13 to the source of negative potential 12. s

As this large current flows through the resistive device or resistive lamp 11, the resistive device 11 heats and the impedance of the device 11 increases. As the impedance of the resistive device 11 increases, an increasing amount of current bypasses the resistive device 11 by flowing from the junction through the resistive element 44, the L:

te'rmined time.delay,'the predetermined magnitude of current passesthrough the relay coil 41 to operate the relay to closethe normally open pair of contacts '46 connected in a circuit to shunt out the thermistor 44. This allows the thermistor 44 to cool and increases the impedance of the thermistor 44 in preparation for the succeeding cycle of operation. The operation of the relay coil 41 also opens the pair of normally closed contacts 47 to disconnect the resistive device 11. Thus, the resistive device 11 is bypassed through the low resistance bypass circuit including the junction 15, the closed pair of contacts 46 of the relay coil 41, the parallel combination of the relay coil 41 and the alarm 42, and the junction 14.

In order to restore the protector circuit for further operation, the normally closed circuit breaker 13 is momentarily opened. The momentary opening of the circuit breaker 13 reduces the current flow through the relay coil below the holding current of the relay to open the closed pair of contacts 46 and to close the open pair of contacts 47.

What is claimed is:

1. In a protective circuit for use with a device to be tested and a source of testcurrent:

a first resistive device connecting the source of test current to the device to be tested, said resistive device having an impedance increasing with the magnitude of the current flowing through it;

i a second resistive device connecting said source of test current to the device to be tested, said resistive device having an impedance decreasing with-the magnitude of the current flowing through it; and g means responsive to both apredetermined increase in impedance of said first resistive device and to a predetermined decrease in impedance of said second resistive device for bypassing both said first resistive device and said second-resistive device, and'for open- I ing the 4. impedance of said resistive device and a predeterr'nined decrease in impedance of said thermistor for closing said pair of normally open-contacts to shunt out said thermistor and thereby bypass said resistive device through said coil and said closed pair of tests.- 3. In a protective circuitfor a temperature sensitive electrical component, i I

a source of D.C.- current connected to said electrical component, I g a bypass circuitin'cluding a relay. coil connected in parallel with said electrical component, 1 means for selectively connecting 'said electrical component to an article under test to flow'current from said source through said component "and, said article,

a device having. a negative-resistive characteristic connected in said bypass circuit for initially precluding operation of said relay coil until said current flows for a predetermined lengthof time,

a second bypass circuit connected around said negative resistance device, and

a normally open contact connected in said second bypass circuit which contact is closed by operation of said relay to shunt out said negative resistance device and maintain the bypass circuit around said electrical component.

- electrical test:

2. In a protective circuit for use with a device under s a source of test current, r

a resistive device connectingsaid source of test current to the deviceunder test, said resistive device having an impedance increasingwith the magnitude of the I current flowing through it,

- a thermistor connected to the device under test, said thermistor having an impedance decreasing with the magnitude of current flowing through it,

a pair of normally open contacts connected in parallel with said thermistor, and

a relay coil connected between said source of test current and the parallel combination of the thermistor and the pair ofnormally open contacts, said relay coil responsive to both a predetermined increase in a thermally sensitive resistive lamp;

means for connecting a source of test current to the ar ticle through said lamp to define a first path to limit the test current delivered to the article;

a relay including a coil, a pair of normally closed contacts and a pair of normally open contacts, said contacts operable in response to a predetermined current through said coil;

a thermistor connected in series with said coil and having a negative resistance characteristic and an initial resistance value to initially limit a current through said coil to a value below said predetermined operating current for a predetermined time;

means for connecting the source of test current, to the article through the series connection of said relay coil and saidthermistor to define a second path in parallel with said first path; f'

means connecting said normally closed contact pair in series with said-lamp for interrupting the current through said lamp when said normally closed contacts are operated after said predetermined time, v

' means connecting said normally open contact pair across said thermistor forshortin'g out said thermistor when said normally open contacts are operated after said i predetermined time. N

5. A protection circuit as recited in claim 4, and includs; t 3

alarm means connected across said coil and responsive to" a current through said second path attaining a value intermediate of the" initially .limited value of 2,482,820 9/1949 Wolfs on et a1. $23 6 9 XR JOHN w. CALDWELL, Primary Examiner DANIEL K; MYER, Assistant Examiner Us. or. X.R. 317-41 

